This invention relates in general to cathode ray tube (CRT) defelection systems and particularly to a vertical size compensation circuit for a CRT.
In conventional flyback type CRT deflection systems, horizontal and vertical oscillators in the respective deflection circuits provide deflection voltages and currents to a deflection yoke that is situated on the neck of the CRT. The electromagnetic field established by the yoke is effective for scanning the electron beam across the face of the CRT to produce a raster. High voltage means are also incorporated for developing the high direct current potential required for operation of the CRT. As is well known, the effectiveness of the electromagnetic deflection signals in deflecting the electron beam (and hence the size of the resulting raster produced) is related to the high voltage, which in turn is a function of the CRT beam current. The problem is that as the high voltage fluctuates, the raster size fluctuates in an inverse relationship since the effectiveness of the deflection signal is changed. Many circuits in the prior art use regulators for attempting to maintain the high voltage applied to the CRT constant.
The present circuit is directed to a compensation circuit for maintaining the vertical size of the raster substantially constant despite changes in beam current loading of the CRT. This is readily and simply accomplished in the invention by sensing the high voltage energy (beam current) supplied to the CRT and varying the discharge rate of the vertical timing capacitor in accordance therewith to proportionally change the peak to peak vertical deflection voltage.